1. Field of the Invention
The present invention relates to a crosstalk removing device for use in a recorded information reproducing apparatus for removing crosstalk components from adjacent tracks, from a read signal read from a recording medium.
2. Description of Related Art
FIG. 1 illustrates the configuration of a recorded information reproducing apparatus.
Referring specifically to FIG. 1, a pickup 100 is equipped with three reading elements "a"-"c". The respective reading elements "a"-"c" simultaneously read recorded signals from three recording tracks formed adjacent to each other on a recording disc 3, and supply the resultant read signals to corresponding head amplifiers 4a-4c, respectively.
As illustrated in FIG. 2, when the reading element "b", for example, is reading recorded information from a recording track T on the recording disc 3 by irradiating the recording track T with a beam spot PB, the reading element "a" reads recorded information from a recording track (T+1), which is a track adjacent to the recording track T, by irradiating this recording track (T+1) with a beam spot PA. Also, in this event, the reading element "c" irradiates a recording track (T-1), which is another track adjacent to the recording track T, with a beam spot PC to read recorded information from the recording track (T-1).
The head amplifiers 4a-4c amplify the read signals respectively supplied from the reading elements "a"-"c"to respective desired levels, and supply the resultant amplified read signals to A/D converters 5a-5c, respectively. The A/D converters 5a-5c sequentially sample the amplified read signals corresponding thereto at the timing of a clock signal supplied thereto from a PLL circuit 10. The resultant read sample value sequences SA-SC are respectively supplied to a crosstalk removing circuit 40.
The PLL circuit 10 detects a phase error possibly implied in the read signal based on the read sample value sequence SB, and generates a clock signal having an oscillating frequency corresponding to the amount of a phase error, and supplies the clock signal to the A/D converters 5a-5c and the crosstalk removing circuit 40, respectively.
The crosstalk removing circuit 40 applies adaptive signal processing utilizing, for example, an LMS (least mean square) adaptive algorithm to the read sample value sequences SA-SC to produce a crosstalk removed read sample value sequence P, which is free from crosstalk components from both the tracks (T+1,T-1) adjacent to the track T, from the read sample value sequence SB read from the track T of FIG. 2.
A Viterbi decoder 30 produces the most likely binary reproduced data based on the crosstalk removed read sample value sequence P.
FIG. 3 is a block diagram illustrating an exemplary internal configuration of the crosstalk removing circuit 140.
Referring specifically to FIG. 3, variable coefficient filters 110, 111, 112 each comprise a transversal filter as illustrated in FIG. 4.
The illustrated transversal filter consists of serially connected n-stage D flip-flops D1-Dn for sequentially shifting and fetching read sample value sequences (SA, SB, SC); a coefficient multiplier MO for multiplying the read sample value sequences by a filter coefficient C.sub.0 ; coefficient multipliers M1-Mn for multiplying respective outputs of the D flip-flops D1-Dn by filter coefficients C.sub.1 -C.sub.n, respectively; and an adder AD1 for adding the respective products from the coefficient multipliers M0-Mn and outputting the sum.
The variable coefficient filter 110 filters the read sample value sequence SB with filter coefficients BC.sub.0 -BC.sub.n supplied from a filter coefficient calculating circuit 123 to produce a read sample value sequence R which has inter-symbol interference removed therefrom, and supplies this read sample value sequence R to a subtractor 120. The variable coefficient filter 111 filters the read sample value sequence SA with filter coefficients AC.sub.0 -AC.sub.n supplied from the filter coefficient calculating circuit 123 to produce a crosstalk sample value sequence CR1 which corresponds to a crosstalk component from an adjacent track (the track T+1 in FIG. 2), and supplies this crosstalk sample value sequence CR1 to the subtractor 120. The variable coefficient filter 112 filters the read sample value sequence SC with filter coefficients CC.sub.0-CC.sub.n supplied from the filter coefficient calculating circuit 123 to produce a crosstalk sample value sequence CR2 which corresponds to a crosstalk component from the other adjacent track (the track T-1 in FIG. 2), and supplies this crosstalk sample value sequence CR2 to the subtractor 120.
The subtractor 120 subtracts the crosstalk sample value sequences CR1, CR2 from the read sample value sequence R to produce the aforementioned crosstalk removed read sample value sequence P which is supplied to each of the Viterbi decoder 30 and a reference sample extracting circuit 130 in FIG. 1.
The reference sample extracting circuit 130 extracts a predetermined reference sample value from the crosstalk removed read sample value sequence P, sequentially supplied thereto from the subtractor 120, and supplies the predetermined reference sample value to a subtractor 140. For example, when the values of three successive sample sequences within the crosstalk removed read sample value sequence P transit from positive to negative or from negative to positive, the reference sample extracting circuit 130 extracts the central sample value of the three successive sample values, i.e., a sample value at zero-cross time, and supplies the extracted sample value to the subtractor 140. The subtractor 140 calculates the difference between the sample value extracted by the reference sample extracting circuit 130 and a predetermined reference value, and supplies the filter coefficient calculating circuit 123 with the difference as an error value "e". For example, when the reference sample extracting circuit 130 extracts a sample value at zero-cross time from the crosstalk removed read sample value sequence P, the reference value presents zero.
The filter coefficient calculating circuit 123 calculates filter coefficients AC.sub.0 -AC.sub.n based on the read sample value sequence SA and the error value "e", and supplies these filter coefficients AC.sub.0 -AC.sub.n to the variable coefficient filter 111 as filter coefficients C.sub.0 -C.sub.n for the variable coefficient filter 111. Likewise, the filter coefficient calculating circuit 123 calculates filter coefficients BC.sub.0 -BC.sub.n based on the read sample value sequence SB and the error value "e", and supplies these filter coefficients BC.sub.0 -BC.sub.n to the variable coefficient filter 110 as filter coefficients C.sub.0 -C.sub.n for the variable coefficient filter 110. Further, the filter coefficient calculating circuit 123 calculates filter coefficients CC.sub.0 -CC.sub.n based on the read sample value sequence SC and the error value "e", and supplies these filter coefficients CC.sub.0 -CC.sub.n to the variable coefficient filter 112 as filter coefficient C.sub.0 -C.sub.n for the variable coefficient filter 112. The filter coefficient calculating circuit 123 repetitively updates each of the filter coefficients AC.sub.0 -AC.sub.n, BC.sub.0 -BC.sub.n, CC.sub.0 -CC.sub.n based on an LMS (least mean square) adaptive algorithm such that the error value "e" converges to zero.
With the configuration as described above, the crosstalk removing circuit 40 first applies the read sample value sequence SB with adaptive signal processing utilizing the LMS adaptive algorithm to eliminate inter-symbol interference from the read signal read from the track T illustrated in FIG. 2 to produce a read sample value sequence R which is free from the inter-symbol interference. Further, the crosstalk removing circuit 40 applies the adaptive signal processing to the read sample value sequences SA, SC to produce crosstalk sample value sequences CR1, CR2 corresponding to crosstalk components from both the tracks adjacent to the track T (T+1, T-1), respectively. Here, the crosstalk sample value sequences CR1, CR2 are subtracted from the read sample value sequence R to produce the crosstalk removed read sample value sequence P which is free from the influence of the crosstalk from the adjacent tracks.
However, if the PLL circuit is out of synchronization to cause a synchronization error among the respective read sample value sequences SA-SC, the reference sample extracting circuit 130 no longer correctly extracts the reference sample. This causes the filter coefficient calculating circuit 123 and the variable coefficient filters 110-112 to perform malfunctions, resulting in the crosstalk removing circuit 40 failing to normally function. Particularly, once this malfunction occurs, even if the PLL circuit 10 recovers its synchronized state, it takes certain time for the filter coefficient calculating circuit 123 and the variable coefficient filters 110-112 to proceed to their normal operations, thus giving rise to a problem that these circuits cannot correctly reproduce data until the synchronized state is recovered.